The purpose of this study was to investigate the dimensional changes and phase transition of nanotubular titanium oxide arrays after heat treatment under an oxygen-containing atmosphere. The thermodynamic background for the oxidation of titanium to titanium oxide was theoretically investigated as a function of the oxygen partial pressure. The anodized titanium nanotubes had lengths between 400 and 500 nm, thicknesses of 11 nm and an amorphous structure. The specimens heat-treated at higher oxygen partial pressures preferentially had rutile phase rather than anatase phase. The thickness of the TiO2 nanotubes was increased at a lower oxygen partial pressure.